In environmental, physical, chemical and microbiological studies, filtration is often used to concentrate small particles (e.g. microbial cells) suspended in minute concentration in fluids or gases. In general, it may be necessary to separate and concentrate small particles according to the particle dimension. Further, it is desirable to rapidly and completely capture these small particles on a filter.
Traditional particle filters consisting of cellulose ester are marketed with an average pore size. The user will usually select a suitable filter to use based on the average pore size of the filter but in general, these filters do not have uniform sized pores but have varied pore sizes. The actual pore sizes lie within a scale range of the average pore size. Accordingly, it may difficult to achieve a complete separation of small particles as these particles are trapped only by a fraction of pores equal to or smaller than the stated average pore size but may not be trapped efficiently by pores larger than the average pore size. Particles (such as microbial cells) may also have a tendency to adhere to the cellulose ester filter after being trapped or be trapped deep within the pores of the ceullulose ester filter. In filtration methods further requiring subsequent collection of trapped particles (for concentration and/or for further identification/analysis), the retrieval or elution of the particles from such cellulose may not be complete.
Other filters include thin polymer filters (usually polycarbonate), which have an irregular array of pores on the surface, a fairly low total area of pores and limited overall filter size. These filters are normally employed for single use only and are usually not strong or reliable enough for repeated use.
Particle filters with uniform pore sizes have been described. For example, inorganic microfiltration membranes with a pore size down to 0.1 μm were fabricated using laser interference lithography and silicon micro machining technology. This filtration membrane was fabricated using silicon nitride deposited on a silicon wafer as a support substrate. These membranes were reported to have small flow resistance due to a thickness smaller than the pore size, a smooth surface, high porosity, narrow pore size distribution, large flux, low trans-membrane pressures and relatively insensible to fouling. However, the fabrication involved numerous process steps and the use of expensive equipment results in a high production costs. Further, the thin membrane (<1 μm) allows only low working pressure (<2 bar) and the lack of choice of membrane material (limited to nitride) also limits the application of this filtration membrane.
A polymer through-hole membrane with high aspect ratios may also be fabricated by nano-imprinting using metal molds. The metal molds were prepared by a replication process using an anodic porous alumina template.
With this technique, different resins can be used to make the through-hole membrane with a diameter of 250 nm. However, the use of the alumina template restricts the pore density and pore size of the membrane and the method typically produces membranes with a pore diameter down to 100 nm only. Further, as the membrane is thin, it is readily torn or damaged when being released from the mold. Increasing the thickness of the polymer membrane may result in larger friction and adhesion forces between the membrane and the metal mold and thus it is also difficult to detach the mold from a thicker polymer.
Phase separation micromolding is also a suitable method to prepare a polymeric filter with polyethersulfone (PES) but the fabrication method can be challenging. For example, although chemical resistance and mechanical properties of PES membranes is suitable for filtration, the fabrication method may result in an increase in pore size during the shrinkage stage and membrane folding and tearing during release from the mold. In addition, if a blend of polymers is used, the ratio of the blend is crucial to the integrity of the membrane during release from the mold.
Fabrication of a microfilter using an excimer laser is also possible but the formation of debris and ridges during the cutting process reduces the quality of the filter surface. In addition, the equipment (laser excimer) is expensive and the process is time-consuming.
SU-8 submicrometric filters fabricated by UV interference lithography are suitable for biological applications, but the thinness of the membrane (typically in the order of 100 nanometers) is not suitable for a filtration system where the filter is subjected to substantial pressure.
It is therefore desirable to further improve on fabrication methods of filters.